In the production of organic saturated aliphatic monocarboxylic acids having 6 to 9 carbon atoms by oxidation of the corresponding aldehydes, soluble manganese and copper compounds, such as manganous acetate and cupric acetate, are used in combination as catalysts to provide high carbon conversion and high efficiency of aldehyde to acid. High carbon conversion and high efficiency are essential not only to provide the good yeilds of acids but also to produce relatively small amounts of undesirable by-products and thus avoid recycling unreacted starting materials. The oxidation reaction can be conducted in the liquid phase using a single stage or multiple stage reactors. These reactions can be operated under pressure in the range from about 60 to about 150 pounds per square inch gauge, preferably from about 85 to about 95 pounds per square inch gauge, with air or oxygen containing gas in the temperature range from about 50.degree. C. to about 80.degree. C. Such a process is described in copending application U.S. Ser. No. 065,241 filed Aug. 9, l979 assigned to the same assignee and filed concurrently with this application.
However, it has been found that metallic copper will precipitate from acids prepared using such mixed soluble catalyst compounds due to reduction during the processing of these acids, especially during distillation. The presence of precipitated copper particles in turn can lead to serious mechanical problems such as reboiler fouling and erosion of pump impellers. Manganese compounds, on the other hand, tend to remain soluble and are not readily reduced to the metal.
Removing the soluble metal catalysts from the acid products would not only avoid the aforementioned mechanical problems but would also provide purer acid products. As has been described in copending application U.S. Ser. No. 065,240 filed Aug. 9, 1979 U.S. Pat. No. 4,246,185 assigned to the same assignee and filed concurrently with this application, the copper and manganese from soluble catalysts can be precipitated from organic saturated aliphatic monocarboxylic acids having 6 to 9 carbon atoms by the addition of aqueous oxalic acid. This procedure utilizes water as a separation means for the precipitated metal oxalates, since the organic acids, being substantially insoluble in water, form a separate organic phase from the aqueous phase containing the precipitated metal oxalates. The resulting organic acids which have been rendered substantially free of manganese and copper then can be readily decanted from the aqueous phase, and need not be filtered. However, using aqueous oxalic acid to remove copper and manganese from lower organic saturated aliphatic monocarboxylic acids containing 2 to 5 carbon atoms is not economically feasible since these acids are soluble in water, and following the addition of the aqueous oxalic acid the resulting solution would have to be distilled to recover the desired acid products.
In the prior art, there are various techniques to describe the removal of metal catalysts from the reaction product. In U.S. Pat. No. 3,840,469, there is a disclosure for the cobalt catalyst recovery from an acetic acid medium derived from the liquid phase oxidation of aliphatic hydrocarbons. This procedure precipitates the cobalt as cobalt oxalate in the acetic acid product. The patent indicates that manganese would not undergo precipitation in this procedure. In U.S. Pat. No. 2,380,731, a procedure is described using oxalic acid to remove numerous metals such as iron, magnesium, chromium, copper, vanadium etc. from a refractory inorganic support such as a clay or alumina or silica or an alumina-silica refractory catalytic cracking catalyst. These procedures are not related to the process of this invention.